Garry's Mod mapping is very similar to HL2 mapping (if not identical), unless you're using custom entities specified for G.MOD (Garry's Mod) only, in which case would make the map unique to gmod.

Steps

1

Open up Steam, and go to the Tools tab.

2

Open Source SDK.

If this is your first time running Source SDK for Garry's mod, you won't see "Garry's Mod" as an option. Look under tips to find out how to get it to show up. You can select HL2 or HL2DM instead.

3

Get familiar with the tools you'll be working with.

Selection Tool - The selection tool is helpful as it has the ability to select objects, entities, and so on (hence the name). Pretty much self-explanatory.

Magnify Tool-- Once again; another self-explanatory tool. The Magnify Tool isn't important. Using the mouse wheel while using the Camera tool zooms in and out just as much as this tool does.

Camera Tool-- This tool is helpful as it allows the mapper to get a 3D perspective of his map (You can change between what the Camera tool renders in the View menu bar.) Left click to alter where the camera is facing, and right click to strafe the camera. If you look at one of your 3 grids while using the Cam tool, you can left click then drag the mouse to set a perspective point.

Entity Tool-- One of the most important tools; the Entity Tool allows you to place entities such as info_player_start ~ player spawns. To access the list of entities, when you click on the Entity Tool, a list on the lower right side of your Hammer should say "Categories: Entities" click on the drop down menu below that and you'll find a list of entities to spawn. Most commonly used are info_player_start, light, light_environment, prop_physics, etc.

Block Creation Tool-- The BC Tool is the most important tool in Hammer, as it is what allows you to create your maps. It is used to create the 'hull' of the map.

Texture Application Tool-- This tool is useful because it lets you target a texture on a surface rather than the entire object's texture. You can also replace textures, rotate, and scale them. The 'Displacement' tool is used for making hills and such.

Apply Texture Tool-- Self-explanatory- it just changes anything you've selected with whatever texture you currently have in your upper right texture menu.

Apply Decals Tool--This tool is for applying decals, which is semi-important. Decals help 'beautify' a map (posters, spray paint, etc). To use it, click on the selection tool and then click on the Apply Decals tool, then while looking at a wall you want to apply a decal to, click on it in the 3D render window and it should pop up on that wall.

Apply Overlay Tool-- I have never used this tool so its *obviously* not very important. It takes the same overlay features (texture application features) and applies them to the selected surface.

Clipping Tool-- This tool is one of the two coolest tools. The clipping tool allows you to clip walls and ultimately make them not rectangles at all as how they were spawned. It's really sweet. You should try it.

Vertex Manipulation Tool-- Here's the second sweetest tool. You need something done; the Vertex Manipulation tool can do it for you. It allows you to manipulate vertexes.

Warnings

Source SDK is a steam tool used for making custom content, and is designed for professional users.

It is important that you close off your map from the rest of the mapping space. Failing to do so will result in a map leak and failed compile of your map, this means that when you start up the map in Garry's Mod it will either crash while your loading it or it won't run at all.